ProjectSummary Aging has a dramatic effect on regeneration of all tissues, bone included. When a young adult fractures a bone, stem cells residing within the bone marrow cavity proliferate and differentiate into osteoblasts. This process results in regeneration of the skeletal element with complete restoration of its biomechanical properties. In the elderly, however, this process is often inadequate, which will result in a delayed healing responseandsubsequentmedicalcomplications. Chronic inflammation mediated by activation of the NF-?B pathway has recently been linked to a decline in regenerative potential in aged skin, skeletal muscle and nervous system, although the direct effects of NF-?B activation on stem cell function in these tissues is still under investigation. Accumulation of senescent cells in aging tissues has been suggested as the driver of chronic inflammation, as these cells secrete inflammatory factors,whichaffecttheentiresurroundingcellularmilieu,furtherstimulatinginflammation. The mechanisms underlying this age-related decline in stem cell function are the focus of this proposal. We hypothesizethattheaccumulatedeffectsofchronicinflammationleadtoosteoprogenitorcelldysfunction. Withinthisproposal,wewillquantify and qualify age-dependent alterations in progenitor cell frequency, proliferative and osteogenic differentiation potential, and determine if these changes are caused by chronological aging or age-associated inflammation.Whilepreviousstudieshavedemonstratedareduced regenerativepotentialofheterogeneousbonegraftsfromagedanimals,thisstudywillforthefirsttimeassess theaffectofagingonahomogeneousprogenitorcellpopulationinaninvivoenvironment. In the second part of this proposal, we will investigate the underlying mechanism of action that is responsible for increased osteoprogenitor cell senescence and resulting regenerative dysfunction. Here, we will identify the machinery that activates telomere dysfunction and cell senescence. From this experiment, we will learn, whether chronic inflammation is the culprit for reduced regenerative function of the agedboneprogenitorcell,andwhetherthisprocessisreversible.Finally,wewillstudytheeffectofchronic inflammation on the regenerative decline of the aging osteoprogenitor cell in a regenerative context andwillutilizetheinformationgainedinthepreviousaimstodesignatherapeuticapproachthatwillleadtothe rejuvenationofskeletalprogenitorcells. The goal of this highly translational project is to identify the basis of the age-related decline in osteogenic progenitor cell function and thus to define a new therapeutic target for improved bone healing in the elderly patient.